The set rrange command sets the range of the radial coordinate for a graph
in polar mode. This has the effect of setting both xrange and yrange as well.
The resulting xrange and yrange are both [-(rmax-rmin) : +(rmax-rmin)].
However if you later change the x or y range, for example by zooming, this does
not change rrange, so data points continue to be clipped against rrange.
Unlike other axes, autoscaling the raxis always results in rmin = 0.
The reverse autoscaling flag is ignored.
Note: Setting a negative value for rmin may produce unexpected results.
